The invention relates generally to combustion cold spray and, in particular, to the apparatus for and methods of combustion cold spray.
Bonded surface layers are desired for many applications including those in which the surfaces experience corrosion, erosion, or high temperature. Bonded surface layers can be produced through cladding processes in which a metal feedstock is melted along with the surface layer of the substrate and resolidified to produce a bonded attachment interface. Cladding processes, particularly for high temperature alloys, can be time-consuming processes that entail considerable expense, and they require a substantial heat input into the part.
Another method used for producing bonded metallic coatings on substrates is cold spray technology. In cold spray technology (also referred to herein as simply “cold spray”), particles are mixed with a gas and the gas and particles are subsequently accelerated into a supersonic jet, while the temperature of the gas and particles is maintained at a sufficiently low temperature to prevent melting of the particles. Copper coatings have been deposited using cold spray in which sufficient bonding was achieved to produce bulk-like properties. However, higher temperature materials such as stainless steel, nickel, nickel-based superalloys and titanium alloys, are likely to require higher velocities to produce high quality deposits with limitations of conventional cold spray devices. In particular, achieving higher particle and deposit temperatures would be desirable.
Bonded deposits produced from particle deposition processes would be more economical than cladding processes, and would enable near net shape forming at a high deposition rate. Cold spray deposition processes are currently limited in the degree of particle consolidation because of temperature limitations in the gas. In order to attain better properties using higher melting point metals than copper, cold spray equipment is moving toward higher gas temperatures.
Combustion thermal spray devices are currently used to produce metallic coatings through particle melting or partial melting and acceleration onto a substrate. They use a combustion process to produce gas temperatures above the melting point of the particles and gas pressures to impart velocity to the particles.
One common problem encountered in the combustion thermal spray process is the susceptibility of the sprayed metal powder to oxidation. It is important to reduce the amount of oxygen present in the metal coating to improve the formability of the coating, to make the coating less brittle, and to improve corrosion resistance. Some normally used methods to reduce the oxygen content in the coatings include thermally spraying the metal powder in a chamber filled with an inert gas, such as nitrogen, and using an inert gas shroud to protect the molten powder from oxidation during the thermal spray process.
Therefore, there is a need for creating a bonded deposit that is more cost-effective than cladding and able to produce higher quality deposits more economically than cold spray for high temperature metals. Further, it is desirable to have a method of depositing high quality metal coatings that result in the metal coating having a low oxygen level without requiring the additional step of annealing.